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Saturday, March 1, 2014

Henri Becquerel and Radioactivity

Henri Becquerel in his lab

On March 1, 1896, French physicist Henri Becquerel while experimenting with X-rays and photographic plates discovered radioactivity along with Marie Skłodowska-Curie and Pierre Curie, for which all three won the 1903 Nobel Prize in Physics.

Henri Becquerel was born into a family of scientists. His grandfather Antoine César Becquerel was a pioneer in the study of electric and luminescent phenomena. His father Alexandre-Edmond Becquerel studied the solar spectrum, magnetism, electricity, and optics. The third generation physicist in the family, Henri Becquerel studied engineering at the École Polytechnique and the École des Ponts et Chaussées. He occupied the physics chair at the Muséum National d'Histoire Naturelle in 1892 and became chief engineer in the Department of Bridges and Highways two years later.

Becquerel dedicated his early works in physics to the plane polarization of light, with the phenomenon of phosphorescence and absorption of light by crystals, which was also the subject of his doctoral thesis. However, his interest in the emission of light of one color following a body's exposure to light of another color, phosphorescence, developed early as well. Becquerel assumed that that phosphorescent materials, such as some uranium salts, might emit penetrating X-ray-like radiation when illuminated by bright sunlight. He was highly influenced and motivated by the huge wave of excitement, that established in the scientific community, after Wilhelm Conrad Röntgen discovered X-rays in the 1890s. Becquerel first heard about Roentgen’s discovery in January 1896 at a meeting of the French Academy of Sciences. After learning about Roentgen’s findings, Becquerel began connecting the two fields of research and started preparing experiments.

He wrapped photographic plates in black paper so that sunlight could not reach them. Then, Becquerel placed the crystals of uranium salt on top of the wrapped plates, and put the whole setup outside in the sun. When he developed the plates, he saw an outline of the crystals. He also placed objects such as coins or cut out metal shapes between the crystals and the photographic plate, and found that he could produce outlines of those shapes on the photographic plates. As a result, Becquerel assumed that his idea was correct and that the phosphorescent uranium salts absorbed sunlight and emitted a penetrating radiation similar to x-rays. On February 24, 1896, he submitted his research results the French Academy of Science, but longed for more confirmation of what he had found. He continued his experiments, even though the weather started to change against his favor (as he thought). Disappointed about the situation, Becquerel put his uranium crystals and photographic plates away in a drawer, which he reopened on March 1. To his big surprise, the image was now amazingly clear. The day after, Becquerel reported at the Academy of Sciences that the uranium salts emitted radiation without any stimulation from sunlight.

This accidental discovery belongs to one of the most important findings in physics, but until this day it it not clear, why Becquerel even developed the plates even though he expected no good result. Howeber, two months later, he figured out that it was indeed the element uranium that emitted the radiation and that his rays could be deflected by electric or magnetic fields. In the following years, Marie and Pierre Curie figured out how to measure the intensity of the radioactivity, and soon found other radioactive elements: polonium, thorium and radium. Furtherly, the rays were separated into alpha, beta and gamma radiation by Ernest Rutherford. In 1903, Becquerel and the Curies shared Nobel Prize for their work on radioactivity.